Method of dewatering slurries using leaf filters (filter presses)

ABSTRACT

The invention relates to a process for dewatering sewage sludges using plate filters, wherein the flocculated sewage sludge treated by means of organic polymer flocculation aids and optionally other auxiliary agents is separated from the major amount of water of the flocculated sewage sludge on a plate filter by means of hydrostatic filtration, whereafter the partially dewatered sewage sludge is subjected to pressure filtration. The plate filters employed have filtrate outlet systems permitting filtrate discharge at a flow rate of 1.0 m/s at maximum. By conducting the process in this way, the dewatering capacity is substantially increased.

BACKGROUND OF THE INVENTION

The invention relates to a process for dewatering sludges, preferablymunicipal sewage sludges in sewage treatment with rapid pressurefilters, particularly chamber filter presses and membrane filterpresses, using water-soluble polyelectrolytes as flocculation aids insludge conditioning.

In addition to agricultural utilization, the obligatory disposal ofsewage sludges is effected by thermal treatment of the sewage sludgewith combustion or degassing and gasification, and predominantly vialandfill.

Sewage sludges represent compositions including exceedingly high levelsof water, so that both in landfill and in thermal processes, the solidscontent of the sludge is increased by at least one mechanical dewateringstage, preferably using screen belt presses, centrifuges or pressurefiltration. Particularly in improving such dewatering on filter presses,the sludge preconcentrated by thickening has to be conditioned by meansof an additional treatment. To this end, the sewage sludge isflocculated using organic, polymeric flocculation aids, preferably highmolecular weight cationic polyelectrolytes, and subsequently subjectedto dewatering in a batchwise procedure using chamber filter presses ormembrane filter presses.

According to EP-B-19176, dewatering can be performed in two steps.Initially, the flocculated sludge is subjected to dewatering in ascreening drum or on separate screening belts and collected in anintermediate container, subsequently fed into the chamber filter pressusing a piston membrane pump or an eccentric screw pump, and filtratedunder a pressure of up to 15 bars. Owing to the previous dewatering,this process is remarkable for its shorter batch times in the chamberfilter press. However, this advantage is compensated by the measures andtime required for the first step of the process.

In gfw-wasser/abwasser 126 (1985), pp. 124-130, V. Zees, T. Clausdorfand G. Gerardts make the supplementary statement that following sludgeconditioning and removal of supernatant water, only limited dewateringof the flocculated sludge in the chamber filter press is possiblebecause no sufficient stability of the flocs is achieved for thispurpose.

According to K. J. Thomé-Kozmiensky, Klärschlammentwässerung, TK-VerlagK. J. Thomé-K., Neuruppin, 1998, p. 266, polymer conditioning of sewagesludge for dewatering on filter presses is advantageous only in thosecases where high demands with respect to shear strength are not made.

Therefore, a dewatering process is known according to EP-A-151,747wherein the flocculent is metered to the conveyed amount of sludge as afunction of sludge density and the sludge thus pretreated is feddirectly into the chamber filter press. With pressure times of 3 hours,a dewatered sewage sludge having a dry matter content of 30-40 wt.-% isachieved in the press discharge.

However, this procedure involves the disadvantageous process that thewater which is difficult to remove from the sludge flocs is notpredominantly withdrawn during pressure filtration and also, thoseamounts of water in the flocculated sludge mixture already made free ofturbidities by flocculation undergo filtration through a developinglayer having a filtering effect and compacting under pressure exposure.Consequently, this dewatering process on chamber filter presses which iswidely used for sewage sludges involves considerable technical andeconomical drawbacks.

DE-OS-38 20 110 describes a process wherein the conditioning of sewagesludge is performed using at least two organic polymer flocculation aidshaving low and high molecular weights, respectively. Again, partialdewatering of the flocculated sludge is possible upstream the chamberfilter press.

According to WO 88/03048, previous dewatering is effected on screenbelts arranged in a story-like fashion, by means of which the sludge isconveyed to a sludge silo without the use of pumps.

Furthermore, a process is known according to U.S. Pat. No. 4,861,492,wherein sewage sludge is flocculated using polymeric flocculation aids,sedimented, and subsequently held in a rest phase without mechanicalload for at least 2 minutes. Thereafter, the supernatant waste water isdecanted, and the thickened sludge is subjected to dewatering in a rapidpressure filter with additional flocculant aid. This process requiresadditional equipment-related input for decanting, as well as time andmeans to stabilize the flocculated state.

In another process, according to Chem. Ing. Techn. 66, No. 9 (1994), pp.1222, 1223, the sewage sludge previously added with flotation coal andashes is subjected to a pressure flocculation in a pressure-resistantflocculation reactor; the flocculated sludge is to reach the filterpress without further mechanical stress, and minimum filter dragthroughout the feeding cycle is to be ensured.

In 44^(th) Purdue Industrial Waste Conference Proceedings, 1990, LewisPublishers, Inc., Chelsea Mich. 48118, 1989 (1990), pp. 513-518, J. T.Shah reports on the possibilities of optimizing the dewatering processon chamber filter presses by stepwise pressure increase during thedewatering process, thus enabling an increase of the filter cake solidscontent.

DE 93 07 712 U1 describes a special design of a sludge dewatering presswherein the pressurized air used in secondary pressing subsequently isutilized to blow out the dewatered sludge. By using this sludgedewatering press, the high consumption of pressurized air is intended tobe reduced.

DE 93 20 903 U1 describes a filtering means for liquids, particularlyswimming pool water, where the liquid to be filtrated is sucked out of acontainer over filtering elements with precoat filter layer. The designof this filtering means is intended to improve the cleaning operation,i.e., removal and disposal of the consumed filtering aid, and reduce theamount of water required for this purpose.

DE 36 17 519 A1 describes a process and a device for removing residualimpurities from a pre-clarified potable liquid, wherein a filter presshaving precoated layers of filtering aids is used. Here, the structureof the regeneratable layer of filtering aids is regarded as crucial tothe invention, precoating with said layer of filtering aids beingeffected at a specific flow rate. Filtration in the filter press isperformed under pressure as usual.

DE 41 19 167 A1 describes a method of determining the characteristicsand parameters essential for the sizing and dimensioning of filterpresses, as well as a device for performing said method. It is ameasuring procedure wherein a hydrostatic pressure corresponding to theprevious filtration pressure is applied in the filter chamber, whichhydrostatic pressure is continuously monitored in order to detect thepressure drop characteristic of completion of the filtration process.

U.S. Pat. No. 5,275,740 describes a method and a device for controllingthe sludge pressure in a filter press and, in particular, forcontrolling the final pressure of the stepwise and automatic filtrationcycles in order to have simultaneous optimization of the press feedingprocess and to maintain a constant pump pressure. In this case as well,filtration is effected in the press under pressure and in a waypreviously known from prior art.

JP 07124600 A describes a process and a device for the treatment oforganic sludges, wherein the organic sludge initially is flocculatedusing an organic polymer flocculant and subsequently subjected todewatering on a filter press using gravity filtration, i.e., solely bythe hydrostatic pressure of sludge supplied to the filter press.

Depending on type and amount of organic and mineral components,municipal sludges exhibit diverse, frequently varying propertiesaffecting the dewatering behavior, particularly during flocculation ofdispersed solids by means of organic flocculation aids and duringfiltration. Particularly in the dewatering of municipal sludges onchamber filter presses, the individual operations of the processtherefore have to be optimized and balanced. In addition to selectingsuitable flocculation aids and apparatus, the dewatering processconsequently is controlled by computer-controlled systems where theflocculation aid is metered to the sewage sludge pump at the pressure orsuction side, and flocculation is optimized using a floc probe andmixing energy control. Also, with currently common initial feedcapacities of 0.15-0.30 m³ of sludge per m² filter area and hour,conveying of sludge to feed the chamber filter press during the fillingphase and the pressure filtration—the latter taking a prolonged periodof time—is controlled electronically in a way so as to avoid anexcessively rapid pressure increase by means of control, because such anincrease would give rise to massive, irreversible compacting of thedeveloping filter cake, thereby affecting the dewatering process. Atpresent, the sludge dewatering capacity of such a process is 6 m³ ofsludge per m³ filter volume and hour or 0.09 m³ of sludge per m² filterarea and hour at maximum.

Accordingly, the well-known dewatering processes are characterized inthat a dynamic pressure is generated in the system even by sludgeconveying of the pumps during the first phase of filtration, whichpressure is increased by an augmenting filtration drag caused bysimultaneous compacting of the developing filter cake as a result ofthis process.

Accordingly, the well-known dewatering processes on chamber filterpresses involve process operations of complicated structure, which alsoare expensive because these processes have to performed separately andin multiple stages.

SUMMARY OF THE INVENTION

It was therefore the object of the present invention to find astraightforward process for dewatering sewage sludge or other sludges,which process would meet the requirements of improved technicalconditions and reduction in costs. In particular, the object was to finda process for removing the water which is contained in the sewage sludgeafter polymer conditioning and free of dispersed solids withoutimpairing the shear stability of the flocculated sewage sludge, andwithout the other drawbacks encountered when performing separate processsteps. The object is supplemented by the additional aspect of optionallyimproving the plate filters as well, namely, in a way so as to besuitable in performing an improved dewatering process, preferably forimproving the plate filter feeding process.

According to ATV-Handbuch Klärschlamm, Ernst & Sohn Verlag Berlin,4^(th) edition, 1996, pp. 357 and 358, the chamber filter pressescurrently employed are constructed in such a way that the filtrate freeof solids is discharged via filtrate discharge channels or collectingchannels in the plate assembly at the bottom and, towards the end offiltration, at the top as well. With high specific throughputcapacities, the hydraulic conditions in the filtrate discharge channels,characterized as drags, represent a limitation, i.e., a bottleneckaccording to Abwassertechnik No. 2, 1991, pp. 44-48.

According to the ATV manual referred to above, chamber filter presseshaving open filtrate outlets are also well-known, where the filtrate isdischarged from the separate plates through lateral outlets, normally onone side. Particularly due to malodors caused by digester gas or ammoniain an optionally required lime conditioning, such chamber filter pressesare not used in sewage sludge dewatering.

It has now been found that dewatering of sewage sludge can be improvedconsiderably according to the process specified in claim 1 where thesewage sludge is subjected to conditioning using at least one organicflocculation aid and optionally other auxiliary agents, and the treatedand flocculated sewage sludge is introduced into the plate filter—alsoreferred to as filter press hereinbelow—which is a chamber filter pressor a membrane filter press or a frame filter press. Therein, hydrostaticfiltration is effected, removing the major amount of water from thetreated, flocculated sewage sludge, whereafter the partially dewateredsewage sludge is subjected to pressure filtration. At least 50 wt.-%,preferably at least 60 wt.-%, more preferably at least 70 wt.-% of thewater is removed in the first step of the hydrostatic filtration.

The invention is based on the finding that, in a hydrostatic filtrationof flocculated sewage sludge, the amount of water of the flocculatedsewage sludge mixtures which is largely free of non-dispersed solids caneasily be removed in a plate filter by introducing the flocculatedamount of solids together with the amount of water into the chamberfilter press wherein these amounts are subjected to said straightforwardfiltration without equipment-related pressure load.

According to the invention, it has also been established that, comparedto the conventional filter press feeding operation, the amount offlocculated sludge conveyed to the filter press can be substantiallyincreased in the initial phase of the filtration process, i.e., duringhydrostatic filtration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In performing the process of the invention, filter presses are used thatare provided with filtrate discharge channels permitting filtratedischarge at a flow rate of 1.0 m/s at maximum. Thus, in particular,plate filters (filter presses) are used where the filtrate can bedischarged through filtrate outlets having large cross-sectional areas,or plate filters having open filtrate outlets or open filtrate outletsystems comprised of single outlets and/or discharge manifolds andoptionally other outlet elements, so that each filtrate can discharge ata flow rate of 1.0 m/s at maximum. Such plate filters are well-known buthave been used for other purposes until now.

Furthermore, plate filters having closed filtrate outlets can bemodified in a way so as to have open filtrate outlets exclusively orpartially or additionally, so that the specified value of the filtrateflow rate would not be exceeded. For example, another improvement of theprocess according to the invention is achieved by optionally enlargingthe cross-section of the filtrate collecting channels formed by thecorner borings of the plates in the plate assembly, and/or by providingadditional openings in least part of the filter plates, preferably inthe lower lateral section of the filter plates, in the vicinity of thefiltrate outlets that are present. Optionally, the discharging filtratecan be passed through open pipes or grooves arranged on the outletopenings. According to the invention, plate filters according to DE 19707 167 A1 are also used in this process.

On the whole, short filtration times and high flow rates of sewagesludge, as well as filtrates free of solids are to be achieved in thebatchwise dewatering of sewage sludge on filter presses. According tothe process of the invention, conveying of the flocculated sewage sludgeduring the feeding phase is performed at an initial feed capacity of atleast 0.3 m³ sludge per m² filter area and hour, preferably in a rangeof 0.35-0.70, and more preferably in a range of 0.40-0.60 m³ sludge perm² filter area and hour. The specified capacity range inevitably resultsfrom the flocculation behavior and the flocculation properties of thesewage sludge and the process-related and equipment-related situationresulting therefrom.

In addition, the process offers the advantage of conveying the flocs ofsewage sludge and the water to the filter press in a gentle fashion.Towards the end of the feeding phase, the amount of flocculated sewagesludge undergoes compacting in the filled chambers as a result of thehydrostatic filtration, thereby generating a pressure increasethroughout the filter press system. During the next phase of pressurefiltration, the feed capacity is reduced to values in a range of below0.3 m³ sludge per m² filter and hour and down to a selected lower value,the so-called turn-off value, with maximum pressure values ranging from10 to 20 bars, preferably from 10 to 15 bars being adjusted in the platefilter.

The dewatering process according to the invention is suitable formunicipal and/or industrial sludges having solids levels ranging from0.5 to 15 wt.-%. Primary, excess and activated sludges, as well asmineralized and preferably, digested sludges, as well as mixturesthereof are employed as municipal sludges. The sludges, normallypreconcentrated in a concentrator, are subjected to conditioning bytreatment with organic polymer flocculation aids, optionally withaddition of inorganic and/or organic builder substances such as lime,iron salts, ashes, filter dust, coal fines, and finely divided parts ofplants, such as sawdust or chopped straw or mixtures thereof.Water-soluble and/or largely water-soluble, partially crosslinkedpolymers, co- and terpolymers of water-soluble non-ionogenic and/orionic monomers and comonomers are used as organic flocculation aids inthe form of a powder, as an aqueous solution, or as a water-in-waterdispersion, or as a water-in-oil dispersion. Such polymers are homo-,co- and terpolymers of monoethylenically unsaturated monomers havingacid groups present at least in part as salts, or their esters withdi-C₁₋₂-alkylamino-C₂₋₆-alkylalcohols or their amides withdi-C₁₋₂-alkylamino-C₂₋₆-alkylamines present in a protonated orquaternized form, such as described in EP-A 113,038 and EP-A 13,416, andoptionally other monoethylenically unsaturated monomers.

Preferably, homo- and/or copolymers of monoethylenically unsaturatedcarboxylic acids and sulfonic acids such as acrylic acid, methacrylicacid, maleic acid, itaconic acid, crotonic acid, and/or their alkalisalts, preferably sodium, potassium or ammonium salts, vinylsulfonicacid, acrylamidoand methacrylamidoalkylsulfonic acids, such as2-acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl methacrylate andstyrenesulfonic acid and/or their alkali salts, preferably sodium,potassium or ammonium salts are suitable as anionic polyelectrolytes,and also, vinylphosphonic acid and styrenephosphonic acid, as well astheir alkali salts, preferably sodium, potassium or ammonium salts.

Preferably, cationically active flocculants, e.g. homo- and/orcopolymers and/or terpolymers of water-soluble, monoethylenicallyunsaturated vinyl compounds, such as acrylic acid esters and methacrylicacid esters of dialkylaminoalkylalcohols in protonated or quaternizedform, such as dimethylaminoethyl acrylate, acrylic acid amides andmethacrylic acid amides of dialkylaminoalkylamines in protonated orquaternized form, such as acrylamidopropyltrimethylammonium chlorideand/or acrylamidopropyltrimethylammonium methylmethosulfate areemployed, preferably together with acrylamide. Copolymers which can beused according to the invention are also described in EP-B-228,637.

The copolymers can be formed of ionic monomers and non-ionogenic,water-soluble, monoethylenically unsaturated monomers, such asacrylamide, methacrylamide, N-C₁₋₂-alkylated (meth)acrylamides, and alsowith N-vinylamide, vinylformamide, N-vinylacetamide,N-vinyl-N-methylacetamide, N-vinylpyrrolidone. In addition, suitablewater-soluble monomers are N-methylolacrylamide,N-methylolmethacrylamide, as well as N-methylol(meth)acrylamidespartially or completely etherified with monohydric C₁₋₄ alcohols, anddiallyldimethylammonium chloride.

Likewise, the copolymers may include limited amounts of ethylenicallyunsaturated monomers sparingly soluble and/or insoluble in water, suchas (meth)acrylic acid alkyl esters and vinyl acetate, as long as thesolubility or swellability of the copolymers in water is retained.

In addition, the polymers can be produced using at least bi-reactivemonomers, preferably diethylenically unsaturated monomers, so as to haveswellability in water or limited solubility therein, or, they may becomprised of water-soluble and water-swellable polymers.

According to the invention, water-soluble or water-swellable amphiphiliccopolymers formed of cationic and anionic monomers and optionallynon-ionogenic monomers may also be employed.

The flocculation aid is selected using flocculation and dewatering testson sewage sludges on a laboratory and pilot plant scale. The agents areemployed in the form of a 0.05-0.5% aqueous practical solution.

In total, the process according to the invention achieves sludgedewatering capacities of 0.1-0.175 m³ sludge per m² filter and hour, ormore than 6 m³, preferably 6-12 m³ sludge per m³ filter volume and hour.

Furthermore, the procedure of the process according to the invention ischaracterized by the advantage of short batch times. A short time periodof a filter press feeding phase is followed by a pressure filtrationphase which is massively reduced in time compared to conventionalprocesses, the time period of which being reduced to 50-20%, preferably35-25%, relative to the previous pressure filtration time, withcomparable values of dry solids content in the filter cake in eithercase, which range from 30 to 42 wt.-% in the process of the inventionwhen exclusively using organic polymer flocculation aids.

The process according to the invention preferably is performed onchamber filter presses or membrane filter presses, the membrane filterpresses achieving additional advantages, particularly a further reducedbatch time, as well as a further increased dry solids content. Relativeto the filter area per plate level, the size of the filter plates usedin the process of the invention is at least 1 m², preferably 1-6 m²,more preferably 1-4 m², corresponding to filter plate dimensions of from1.0×1.0 m to 2.0×2.0 m, for example.

Moreover, the dewatering process of the invention is characterized inthat the sewage sludge is flocculated by said conditioning and partiallydewatered during hydrostatic filtration in such a way that feeding ofthe plate filter with sludge—largely in a controlled fashion atpresent—can be cut down or optionally omitted because the major amountof sludge is introduced into the filter chambers during the hydrostaticfiltration.

The invention will be illustrated in more detail with reference to thefollowing Examples.

COMPARATIVE EXAMPLE 1

Dewatering of municipal digested sludge having a solids content (drymatter) of about 4 wt.-% was performed on a chamber filter press byRittershaus & Blecher Company, type AEHIS, model 1979, having thefollowing characteristics:

Plate size 1.2 × 1.2 m Plate material GGG (grey cast iron) Number ofchambers 115 Filter area 250 m² Total press volume about 3.5 m³

Sludge conditioning and feeding of the chamber filter press wereeffected using the computer-controlled Floctronic® system supplied byStockhausen GmbH & Co. KG. Fa. Following a flow rate measurement using afeed pump controlled by a pressure gage, the digested sludge was pumpedinto the filter press. Addition of Praestol® 853 BC cationicflocculation aid was effected as a 0.1 wt.-% aqueous practical solutionat the pressure side, as was the subsequent mixing energy control by thefloc probe. Initially, the filter press was fed at an initial feedcapacity of 50 m³ sludge per hour, corresponding to 0.2 m³ sludge per m²filter area and hour. With a steady pressure increase to a maximum valueof 15 bars, the feed capacity was adjusted to a value of 10 m³ sludgeper hour by computer control, at which pressure the filtration wascompleted after a filtration time of 88 minutes. From a pressure of 1bar on, 75 minutes of this time was required for pressure filtration. Atotal of 28 m³ sludge was treated with 4.5 kg of Praestol® 853 BC andconveyed into the chamber filter press. The sludge dewatering capacityis 4.2 m³ sludge per m³ filter volume and hour, or 0.06 m³ sludge per m²filter area and hour. The press was opened, and the sludge (filter cake)was obtained with a solids content of 32.6 wt.-%.

EXAMPLE 1

Dewatering of municipal digested sludge having a solids content of about4 wt.-% was performed on a chamber filter press by Rittershaus & BlecherCompany, type AEHIS, having the characteristics specified in ComparativeExample 1. The chamber filter press was provided with an open filtratedischarge system. To this end, the borings in the region of the lowerfiltrate outlet formed by the filter plates, each one provided with ablind plug in the frame of the filter plate in a laterally alternatingfashion, were opened towards the closed filtrate outlet and extended toa nominal width of DN25 (25 mm). In addition, each opening was providedwith outlet tubes about 30 cm in length.

Sludge conditioning and feeding of the filter presses with digestedsludge were performed by means of the apparatus specified in ComparativeExample 1, using the Floctronic® system. Praestol® 853 BC in the form ofa 0.1 wt.-% aqueous practical solution was used as flocculation aid inconditioning. The flocculated, digested sludge was introduced into thefilter press at an initial feed capacity of 120 m³ sludge per hour,corresponding to 0.48 m³ sludge per m² filter area and hour. Withongoing hydrostatic filtration, the major amount of 72 wt.-% of waterfree of solids, relative to the total amount of water of the treated,flocculated sewage sludge, was separated from the sludge mixturedischarging through the open filtrate outlets of the system.Simultaneously, the filter press was continuously fed with conditionedsludge. Once the chambers were completely filled with partiallydewatered sludge, the pressure in the filter press increased to amaximum value of 15 bars as a result of the controlled and increasinglyreduced feeding with sludge, ultimately being 10 m³ sludge per hour.After a pressure filtration time of 24 minutes and a filtration timetotalling 34 minutes, a total of 30 m³ of digested sludge had beenconditioned with 4.3 kg of Praestol® 853 BC and dewatered in the chamberfilter press. The sludge dewatering capacity is 10.3 m³ sludge per m³filter volume and hour or 0.144 m³ sludge per m² filter area and hour.The filter press was opened, and a sludge (filter cake) having a solidscontent of 34.2 wt.-% dry matter was obtained.

The filtration procedure of the invention is characterized by asubstantially reduced batch time and, in particular, a short pressurefiltration phase.

EXAMPLE 2

Dewatering of municipal digested sludge having a solids content of about4.6 wt.-% was performed on a chamber filter press by Rittershaus &Blecher Company, type AEHIS, with the modifications described inExample 1. In addition to the open lateral filtrate outlet system, eachoutlet system above the filtrate channel in the plates was opened with afiltrate outlet having a diameter of DN80 (80 mm) and connected outsidethe chamber filter press to the filtrate outlet pipe system having adiameter of DN300 (300 mm).

Sludge conditioning and feeding was performed as specified in Example 1,and the flocculated sludge was introduced into the chamber filter pressat an initial feed capacity of 120 m³ per hour. After a pressurefiltration time of 21 minutes and a filtration time totalling 31minutes, a total of 30 m³ of digested sludge had been conditioned with4.3 kg of Praestol® 853 BC and dewatered in the chamber filter press.

What is claimed is:
 1. A process for dewatering a flocculated sewagesludge which has been flocculated with at least one organic flocculationaid, said process comprising the steps of: providing a plate filter,modifying the plate filter to permit an initial feed rate of at least0.3 m³ sludge per m² filter area per hour and a filtrate discharge flowrate of at most 1.0 m/second, introducing the flocculated sewage sludgeinto a plate filter, removing at least 50 wt.-% water from theflocculated sewage sludge in the plate filter by hydrostatic filtrationto obtain partially dewatered sewage sludge, and thereafter subjectingthe partially dewatered sewage sludge to pressure filtration in saidplate filter.
 2. A process according to claim 1, wherein in addition tosaid at least one flocculation aid the sewage sludge is conditioned withinorganic and/or organic builder substances.
 3. A process according toclaim 1, wherein at least 60 wt.-% of the water in the flocculatedsewage sludge is removed by hydrostatic filtration.
 4. A processaccording to claim 1, wherein at least 70 wt.-% of the water in theflocculated sewage sludge is removed by hydrostatic filtration.
 5. Aprocess according to claim 1, wherein the filtrate is discharged fromthe plate filter through a closed filtrate outlet system.
 6. A processaccording to claim 1, wherein the filtrate is discharged from the platefilter through an open filtrate outlet system.
 7. A process according toclaim 6, wherein said open filtrate outlet system is comprised oflateral single outlets from individual plates of the filter.
 8. Aprocess according to claim 6, wherein said open filtrate outlet systemfurther comprises at least one collecting channel.
 9. A processaccording to claim 6, wherein said open filtrate outlet system iscomprised of at least one filtrate collecting channel.
 10. A processaccording to claim 1, wherein the plate filter is a chamber filterpress.
 11. A process according to claim 1, wherein the plate filter is amembrane filter press.
 12. A process according to claim 1, wherein thepressure filtration uses a maximal pressure in the range 10 bar to 20bar.
 13. A process according to claim 1, wherein the filter plates ofthe plate filter have an area of at least 1 m².
 14. A process accordingto claim 1, wherein said sewage sludge is comprised of at least onesludge selected from the group consisting of municipal sludge andindustrial sludge and said sewage sludge has a solids content in therange from 0.5 to 15 wt.-%.
 15. A process for dewatering a flocculatedsewage sludge which has been flocculated with at least one organicflocculation aid, said process comprising the steps of: providing aplate filter, enlarging the discharge openings of the plate filter topermit an initial feed rate of at least 0.3 m³ sludge per m² filter areaper hour and a filtrate discharge flow rate of at most 1.0 m/second,introducing the flocculated sewage sludge into a plate filter, removingat least 50 wt.-% water from the flocculated sewage sludge in the platefilter by hydrostatic filtration to obtain partially dewatered sewagesludge, and thereafter subjecting the partially dewatered sewage sludgeto pressure filtration in said plate filter.